1. Field of the Invention
The present invention relates to an optical unit for use in a laser beam printer, and more particularly to an optical unit which guides a laser beam from a laser diode to an object to be scanned, by way of a group of focusing lenses and a scanner.
2. Description of the Related Art
In general, an optical unit incorporated in a laser beam printer or the like is designed such that a laser beam output from a laser diode is guided first to a scanner, i.e., an optical deflector and then to a photosensitive body, i.e., an object to be scanned. Before reaching the photosensitive body, the laser beam passes through a first optical system and a second optical system. The first optical system converges the laser beam generated by the laser diode, while the second optical system focuses the laser beam to a predetermined position on the surface of the photosensitive body. When passing through the first and second optical systems, the laser beam is made to have a cross section of desirable size.
The first optical system is a combination of lenses, such as an aspheric glass lens, plastic lenses, etc. The second optical system is a combination of lenses, such as an f.theta. lens. By this f.theta. lens, the deflection angle at which a laser beam is deflected by the scanner is changed in proportion to the position at which the laser beam is focused on the photosensitive body and which is expressed in relation to the main scanning direction.
In general, the laser diode and that lens of the first optical system which collimates the laser beam generated by the laser diode are assembled together as a laser unit. The lenses of the first optical system other than the collimator lens guide the collimated laser beam to the scanner. These beam-guiding lenses are provided independently of one another, or are assembled together as one unit.
Each of the scanner and second optical system is provided independently of the first optical system, and is individually arranged with reference to a frame member.
In comparison with the scanner and the second optical system, the laser diode and those lenses of the first optical system other than the collimator lens have to be arranged with very high accuracy. Due to this, the kind of the material constituting the frame member is limited, and the working process is also restricted. In addition, the laser diode has an astigmatic range; that is, the laser diode has different apparent light-emitting points between a vertical direction and a horizontal direction. When the optical unit is assembled, therefore, the position of the laser diode must be adjusted such that the beam spot of a laser beam falls within an optimal region determined in relation to both the vertical and horizontal directions. Thus, a lot of time and complicated procedures are required for the assembly and adjustment of the optical unit.